Most conventional wireless communication systems employ the so-called cellular architecture. In this architecture, the coverage area is divided into multiple cells, and users in different cell are served by different base stations. Traditionally, each user communicates only with its serving base station. Signal to/from other base stations are treated as interference.
As base stations are connected to each other with high-speed cables or optical fibers, it is possible to coordinate the transmission and reception of these base stations. In this alternative arrangement with extensive inter-base station coordination, the inter-cell interference can be greatly reduced and system performance can be improved. This is the concept of Coordinated Multi-Point (CoMP) transmission/reception, which was shown to provide tremendous performance gain compared with that without coordination. G. J. Foschini et al., “The value of coherent base station coordination,” in Proceedings of the 39th Annual Conference on Information Sciences and Systems (CISS '05), March 2005 discusses the use of coherent base stations coordination.
Despite some similarities, this kind of coordination should not be confused with traditional MIMO systems. In the latter setup multiple antennas are used for transmission one or more data streams to the user from the same base station site. In the former scenario (i.e. CoMP or DAS) the user receives or transmits from/to multiple base station sites in a well coordinated fashion. Hence although MU coherent coordination schemes in CoMP have the similar principle to MU-MIMO techniques in a traditional cellular system, schemes for these two types of system differ from each other due to different channel conditions of these two systems.
US Patent applications 2007/0223422 and 2007/0223423 disclose a method for scheduling users in a MIMO system. Users are divided into two groups: one for users that may be scheduled together and one for users that are to be scheduled individually.
To explore the benefit of coordination between multiple sites, different methods have been proposed.
One kind of scheme that extensively investigated is coherently coordinated multi-user transmission/reception (MU coherent coordination for short). With this kind of scheme, a system with multiple sites can be treated in a similar way as a network MIMO system. Then techniques similar to traditional MU-MIMO techniques can be applied for both uplink and downlink in this CoMP system. Note that different terminology is used for this type of network arrangement e.g. distributed antenna system (DAS) is another well known term. However they all correspond to similar concept in which transmission and reception from different base station sites are coordinated. In the uplink, MMSE or MMSE-SIC, can be applied to eliminate inter-user interference. Similarly in the downlink, potential inter-user interference can be suppressed before transmission with techniques like ZF beamforming or ZF beamforming with dirty paper coding.
Besides this coherently coordinated multi-user transmission/reception, it is also possible to use other kinds of coordination schemes. These schemes may be categorized into the following two major types:                Coherently coordinated single-user transmission/reception (SU coherent coordination for short):        In these schemes multiple sites jointly serve one user with techniques like beamforming or transmit/receiving diversity. With such schemes, the SNR or BER performance of the severed user is improved.        Non-coherently coordinated transmission/reception (non-coherent coordination for short):        This type of scheme is similar to the idea of inter-cell interference coordination (ICIC) between multiple sites, which may be non-coherent or asynchronous.        
Multi-User (MU) coherent coordination may lead to significant gain compared with that without coordination. However, this kind of coordination requires Channel State Information (CSI) or other relevant downlink quality metric at the network for downlink transmission. This leads to increased uplink signaling overheads as well as increased processing complexity at UEs in average. For uplink transmissions, the complexity at the receiver side is also quite high.